The present invention relates to a microwave push-pull frequency converter of the type described in the Special Reprint from NTZ, Volume 33 (1980), No. 9, pages 590-591.
Low-loss direct converters for television/radio satellites are designed using waveguide techniques. Such a converter converts, for example, five frequency modulated received signals within a 19 GHz band having a width of 400 MHz directly to a transmitted signal of 12 GHz. Other possible frequency combinations for such direct conversion are, for example, 6 to 4, 14 to 4, 14 to 12, 30 to 4, 30 to 20 and 30 to 12 GHz. For spacecraft, semiconductor converters must meet very high demands with respect to undesirable conversion products. They are therefore equipped with input and output filters.
High power undesirable conversion products may be reflected by the filters toward the semiconductor converter elements and may be reconverted and thus contribute to improvement of conversion losses, if reflected in the correct phase. In addition of these positive characteristics, such filters also have negative characteristics. The conversion results in a transmission characteristic with much interference and errors occur in matching. With increasing bandwidth, these secondary effects of the filters become more and more noticeable. For a converter whose input and output frequencies are very far apart, for example 30/12 GHz, filters must be provided which exhibit the lowest possible insertion loss and a very wide blocking range. In the reprint from NTZ, volume 33, (1980) No. 9, pages 590-591, it is proposed to provide a so-called 37 Evanescent Mode Filter" on the low frequency side of the converter. According to IEEE Transactions on Microwave Theory and Techniques, Volume MTT-19, No. 3, March, 1971, pages 295-308, particularly page 302, such asn evanescent mode filter exhibits a parasitic bandpass behavior. It is therefore only conditionally usable in connection with particularly broadband converters.